Enhancing blood flow and glucose uptake in insulin-resistant skeletal muscle

A key aspect of insulin action on muscle glucose uptake is capillary recruitment, and loss of capillary recruitment is thought to contribute to muscle insulin resistance. Thus enhancing insulin-mediated capillary recruitment may improve muscle insulin sensitivity. Acute inhibition of the NO signalling pathway completely blocks capillary recruitment and reduces glucose uptake by 50%. NO signals via cGMP which is hydrolyzed by phosphodiesterases (PDEs). Site specific NO signalling can be enhanced by inhibiting particular PDEs isoforms as shown by the use of Viagra¬¨vÜ in treating erectile dysfunction. PDE inhibition may therefore lead to an increase in capillary recruitment and glucose uptake. Thus the aim of this project was to determine which PDE isoforms are expressed in the muscle vasculature and whether inhibition of these PDEs could enhance glucose uptake and capillary recruitment in normal and insulin resistant rats. Skeletal muscle and aortic tissue were examined for PDE expression at both the mRNA and protein level, and immunohistochemistry used to determine the distribution of PDE expression in the vasculature. PDEs 1B, 5A, 9A, 10A and 11A were found to be expressed in the skeletal muscle vasculature. Zaprinast inhibits PDE isoforms 1, 5A, 9A, 10A and 11A and thus its effects were examined during hyperinsulinaemic euglycaemic clamps in anesthetized rats. Zaprinast moderately increased insulin's effect on glucose uptake and capillary recruitment. To determine the effects of Zaprinast in a model of insulin resistance, a high fat fed (HFF) model of insulin resistance was developed. High fat feeding abolished insulin mediated capillary recruitment and significantly blunted insulin-mediated glucose uptake. In HFF rats, Zaprinast was unable to enhance insulin's metabolic and vascular effects. To further examine the defects in insulin sensitivity and NO signalling in HFF rats, an alternative capillary recruitment agent, methacholine was administered locally into one hindleg by retrograde infusion of the epigastric artery during hyperinsulinaemic euglycaemic clamps. Methacholine was also unable to enhance insulin sensitivity in HFF rats. In conclusion, Zaprinast improved insulin-mediated capillary recruitment and glucose uptake in normal rats. The heterogeneous vascular expression of PDE isoforms indicates that an isoform specific PDE inhibitor may give a greater improvement in insulin sensitivity, particularly in the HFF model where insulin's effects are already blunted. However, the insensitivity of the HFF rats to methacholine and Zaprinast suggests that increasing NO signaling via raised cGMP formation may be difficult due to factors such as NO destruction in HFF rat vasculature.